1. Field of the Invention
The present invention relates to a method for forming contact holes of a semiconductor device, and more particularly to a method for forming contact holes of a semiconductor device, capable of achieving the formation of contact holes having a critical dimension and yet exhibiting an increased tolerance in design rule.
2. Description of the Prior Art
The recent tendency in fabricating highly integrated semiconductor devices is greatly affected by the development of techniques for forming patterns having a critical dimension. Photoresist film patterns formed by a photolithography process are widely used as masks for carrying out an etching process or an ion implantation process in the fabrication of semiconductor devices. In this regard, it is required to achieve a patterning of photoresist film patterns having a critical dimension, a stable performance of process steps, a clean removal of the photoresist film patterns after completion of the process steps, and an easy re-working for forming a new photoresist film pattern after the removal of an inaccurately formed photoresist film pattern.
Generally, a photoresist film pattern is formed by uniformly coating a photoresist solution consisting of a photoresist agent and a resin solved in a solvent in certain amounts over a semiconductor substrate by use of a spin coating process to form a photoresist film, and then selectively irradiating a light onto the photoresist film through a light exposure mask provided with light shield film patterns made of chromium by use of a step and repeat exposure device (hereinafter referred to as "stepper") adapted to repeatedly carry out an alignment and a light exposure, thereby exposing to the light a portion of the photoresist film except for its portion to form the photoresist film pattern. In this case, the light shield film patterns of the light exposure mask have a dimension larger than that corresponding to the resolution of the stepper.
Thereafter, the light-exposed portion of the photoresist film is removed using a weak alkali development solution containing tetra methyl ammonium hydroxide as its base component.
However, this general technique has difficulty in forming a fine pattern having a critical dimension not larger than a predetermined dimension, for example, a pattern having a pattern space not more than 0.4 .mu.m due to various limitations on the accuracy of the light exposure device and wavelength of light.
In other words, the resolution R of the stepper used to fabricate conventional semiconductor devices is in reverse proportion to the numerical aperture of the light exposure device and in proportion to the wavelength of a light source. As a result, the resolution is limited because there are limitations on reducing the wavelength of light and increasing the numerical aperture.
For example, G-line, i-line and excimez laser steppers having wavelengths of 436, 365 and 248 nm have resolutions only capable of forming patterns of about 0.7, 0.5 and 0.3 .mu.m, respectively.
As a semiconductor device has a higher integration degree, its cell size is reduced. This also results in a limited design rule for each thin film. In the case of forming contact holes, the design rule is limited due to the reduced unit cell size and the degraded process capability. Such a limited design rule results in reductions in various design margins such as the contact size, the space between adjacent contacts and the overlap of each contact with a lower layer disposed beneath the contact.
Where contact holes are defined using a positive photoresist film for forming the contact holes having a critical dimension of a semiconductor device, an exposure mask 10 shown in FIG. 1 is used.
As shown in FIG. 1, the exposure mask 10 has a light shield film 12 formed over a quartz substrate (not shown). The exposure mask 10 also has windows 14 each formed by removing a portion of the light shield film 12 corresponding to each contact hole to be formed.
Since each contact hole has a reduced dimension due to a high integration of a semiconductor device, the exposure mask 10 has a reduced dimension a of each window 14. In this case, the space b between adjacent windows 14 corresponding to the space between adjacent contact holes is also reduced.
As the dimension a approximates to an optical limitation by the light exposure device, light beams passing through the exposure mask 10 interfere with one another if the value of b/a is less than 1. In this case, an interference light is generated between adjacent regions where contact holes are defined, as shown in FIG. 2. Due to this interference light, a photoresist film pattern for forming the contact holes is damaged or inaccurately formed. In a severe case, no contact hole is formed. Consequently, the process yield is degraded.